The foam laid process for making non-woven webs as an alternative to the liquid laid process has been recognized as highly advantageous in a number of circumstances. One of the significant advantages thereof is the ability to incorporate into the fiber-foam slurry a wide variety of different types and sizes of particles without significantly adversely affecting the formation of the web. Particularly useful in this regard for some circumstances is the ability to add super absorbent polymer particles or fibers ("SAP") to the fiber-foam slurry. Advantageous methods and systems for utilizing SAP or like particles or fibers in the foam process are described in co-pending applications Ser. No. 08/923,900 filed Sep. 4, 1997, and Ser. No. 08/991,548 filed Dec. 16, 1997 (the disclosures of both of which are incorporated by reference herein). According to the present invention a method and system have been created which make much more versatile the addition of SAP and like particles or fibers to the foam slurry, in the production of non-woven webs therefrom.
A significant problem with the addition of SAP to non-woven webs is that the SAP, by its very nature, quickly absorbs any water that it comes into contact with at ambient conditions. Also because of its sticky nature it can interfere with the web forming equipment (foraminous elements, typically called "wires"). This has greatly restricted the ability to utilize SAPs in a wide variety of products or circumstances, and/or has a required a great energy penalty in drying out the web formed so as to drive the water out of the SAP so that it could perform effectively in the non-woven web produced.
According to the present invention, various techniques for specifically providing or handling the SAP particles or fibers, or the like, have been developed which greatly enhance the versatility of using SAPs in the production of non-woven webs, particularly from cellulose or synthetic fibers such as conventional wood pulp, rayon, polyester, or like fibers. By utilizing one or more of the following techniques, the absorption of liquid by the SAPs can be delayed a sufficient time so that the "dry content" (that is the remaining ability to absorb liquid by the SAP as a percentage of its total ability to absorb liquid) is much higher then when handled conventionally, include: utilizing SAP particles or fibers having a protective coating which dissolves only once in contact with water for a few seconds; adding the SAPs to the fiber-foam slurry by entraining the SAP particles or fibers in a small flow of chilled water (typically at a temperature between about 0-5.degree. C., e.g. 0-3.degree. C., preferably 1.degree. C.), and/or freezing the SAP particles or fibers prior to introduction into the foam-fiber slurry (reducing their temperature to below 0.degree. C., e.g. to the conventional temperature reached by normal freezers or the like, e.g. about -18.degree. C.). Also, the SAP is added to the fiber-foam slurry just prior to web formation (in a headbox, or other device containing the foraminous element or elements), typically about ten seconds or less before web formation starts, and more preferably about five seconds or less (e.g. about three seconds) before web formation is initiated.
The results achieved according to the invention compared to the prior art may be quite dramatic. For example using the conventional prior art techniques where ambient SAP is added to the fiber-foam slurry about ten seconds before web formation is initiated, the dry content of the SAP in the web after web formation is about 10-15%. Using SAP that is reduced in temperature to -18.degree. C., however, and adding it about ten seconds before web formation results in a dry content of the SAP in the non-woven web produced of between about 20-25%. Where water at about 1.degree. C. is used to transport the SAP the dry content in the final web is between about 30-35%, while if the chilled water and the low temperature SAP are combined the dry content of the SAP in the formed web is between about 33-38%. If conventional coated SAP is used and added about ten seconds prior to web formation, the dry content of SAP in the final web produced is about 40%, whereas if coated SAP that is also reduced in temperature to about -18.degree. C. and added with chilled water, is used, the dry content is about 42%. Even higher dry contents can be achieved if the addition occurs about five seconds or less before web formation, and in any event all of the techniques according to the invention result in greatly reduced drying energy and/or time, making the web formation process much more cost effective and simpler for a wide variety of products, including products used in diapers, absorbent pads, and the like.
According to one aspect of the present invention a method of producing a non-woven web of cellulose or synthetic fibrous material (using the foam process) is provided. The method comprises the following steps: (a) Forming a first foam slurry of air, water, cellulose or synthetic fibers, and surfactant. (b) Moving a first foraminous element in a first path. (c) Passing the first foam slurry into operative contact with the first foraminous material moving in the first path. (d) Adding super absorbent polymer to the first foam slurry, and positively mixing it with the first foam slurry, about ten seconds or less before step (c). And, (e) forming a fibrous web from the first foam slurry by withdrawing foam and liquid from the slurry through the first foraminous element. Step (a) is a typical fiber-foam slurry step as described in co-pending application Ser. No. 08/923,900 filed Sep. 4, 1997.
Steps (a) through (e) are typically practiced so that the dry content of the super absorbent polymer after step (e), and before drying, is at least about 20% (typically at least about 30%, and more desirably at least about 40%).
Step (d) may be practiced by adding SAP having a protective coating that dissolves after a few seconds in contact with water, in which case steps (a) through (e) are typically practiced so that the dry content of the SAP after step (e), and before drying, is at least about 35%. Step (d) may alternatively or additionally be practiced by adding SAP at a temperature of below 0.degree. C. (e.g. about -18.degree. C.). Step (c) is typically practiced at a first flow rate, and step (d) may be alternatively or additionally practiced by adding SAP to a flow of liquid water having a temperature between about 0-5.degree. C., preferably between about 0-3.degree. C., (e.g. 1.degree. C.) having a second flow rate, less than about 2% (e.g. less than about 1%) of the first flow rate, and then passing the liquid with SAP into the first foam slurry.
Step (d) is typically further practiced by mechanically mixing the liquid and SAP with the first foam slurry after the liquid and SAP have been added to the first foam slurry, such as by using a conventional mechanical mixer with a rotating blade. Step (d) may alternatively or further be practiced by adding SAP to a second fiber-foam slurry having a solids consistency of between about 5-50%, and pumping the second slurry with SAP into the first slurry, mixing inherently occurring during this pumping. Step (d) is preferably practiced about five seconds or less (e.g. about three seconds) before step (c). There is typically also the further step (f) of drying the web so that the SAP therein has a dry content of at least about 98% (preferably close to 100%). The drying is practiced in a conventional manner, e.g. using a conventional blown hot air system, or conventional drying oven. While the SAP in the final web will pick up some moisture during transport to its final use destination, it typically will have a dry content of at least about 95% when used.
The method of the invention may also comprise the further steps of (g) moving a second foraminous material in a second path; (h) making up a second foam slurry of air, water, cellulose or synthetic fibers, and surfactant; and (i) passing the second foam slurry into direct contact with the second foraminous material; and wherein step (e) is practiced to bring the first and second foam slurries into contact with each other and so that foam and liquid is withdrawn through both the first and second foraminous materials; and also optionally the further steps of (j) making a third foam slurry of air, water, cellulose or synthetic fibers, and surfactant; and (k) moving the third foam slurry directly into contact with the first foraminous material; and wherein step (c) is practiced by passing the first foam slurry between the second and third foam slurries, so that the first foam slurry does not directly contact the foraminous materials. Alternatively step (c) may be practiced by passing the first foam slurry directly into contact with the first foraminous material if build-up problems are avoided because of the entrainment of the SAP in the fiber-foam slurry.
According to another aspect of the present invention a method of producing a non-woven web is provided comprising the following steps: (a) Forming a first foam slurry of air, water, cellulose or synthetic fibers, and surfactant. (b) Moving a first foraminous element in a first path. (c) Passing the first foam slurry into operative contact with the first foraminous material moving in the first path. (d) Adding super absorbent polymer having a protective coating that dissolves after a few seconds in contact with water to the first foam slurry, and positively mixing it with the first foam slurry. And, (e) forming a fibrous web from the first foam slurry by withdrawing foam and liquid from the slurry through the first foraminous element. And, wherein steps (a)-(e) are practiced so that the dry content of the super absorbent polymer after step (e), and before drying, is at least about 35%.
According to yet another aspect of the invention a method of producing a non-woven web is provided comprising the steps of: (a) Forming a first foam slurry of air, water, cellulose or synthetic fibers, and surfactant. (b) Moving a first foraminous element in a first path. (c) Passing the first foam slurry into operative contact with the first foraminous material moving in the first path. (d) Adding super absorbent polymer at a temperature of below zero degrees C to the first foam slurry, and positively mixing it with the first foam slurry. And, (e) forming a fibrous web from the first foam slurry by withdrawing foam and liquid from the slurry through the first foraminous element. And, wherein steps (a)(e) are practiced so that the dry content of the super absorbent polymer after step (e), and before drying, is at least about 20%.
According to another aspect of the invention a method of producing a non-woven web is provided comprising the following steps: (a) Forming a first foam slurry of air, water, cellulose or synthetic fibers, and surfactant. (b) Moving a first foraminous element in a first path. (c) Passing the first foam slurry into operative contact with the first foraminous material moving in the first path at a first flow rate. (d) Adding super absorbent polymer to the first foam slurry, and positively mixing it with the first foam slurry by adding super absorbent polymer to a flow of liquid having a second flow rate, less than about 2% of the first flow rate, and then passing the liquid with super absorbent polymer into the first foam slurry. And, (e) forming a fibrous web from the first foam slurry by withdrawing foam and liquid from the slurry through the first foraminous element. And, wherein steps (a)-(e) are practiced so that the dry content of the super absorbent polymer after step (e), and before drying, is at least about 25%. Step (d) is typically further practiced by using liquid water having a temperature of between about 0-5.degree. C., and step (d) is also further practiced by mechanically mixing the liquid and super absorbent polymer with the first foam slurry after the liquid and super absorbent polymer have been added to the first foam slurry; and step (d) may also be further practiced by adding super absorbent polymer at a temperature of below zero degrees C.; and wherein steps (a)-(e) are practiced so that the dry content of the super absorbent polymer after step (e), and before drying, is at least about 33%.
According to another aspect of the present invention a system for producing a non-woven fibrous web is provided comprising the following components: A first foraminous element on which a non-woven fibrous web may be formed. A first conduit for feeding a fiber containing foam slurry to the first element. A mechanical mixer disposed in the first conduit adjacent the foraminous element. And, an inlet for introducing absorbent polymer into the feed conduit on the opposite side of the mixer from the first foraminous element, to effect mixing of super absorbent polymer and fiber containing foam within the feed conduit.
The system may further comprise a second conduit connected to the inlet, the second conduit having a cross-sectional area not more than 10% of the cross-sectional area of the first conduit. A pump may be provided in the second conduit, and means may be provided for separately feeding SAP and water (or other transporting fluid) into the second conduit on the opposite side of the pump from the inlet.
The system may further comprise a chiller operatively connected to the means for feeding water into the second conduit, the chiller capable of cooling water flowing to the second conduit to a temperature of about 3.degree. C. or less. The means for feeding super absorbent polymer to the second conduit comprises a freezer capable of reducing the temperature of the super absorbent polymer below 0.degree. C., a weighing device, and a metering device.
The system may also include a tank having a fluid level therein and connected to the opposite side of the pump from the inlet; and means for separately feeding super absorbent polymer and fiber containing foam into the tank below the fluid level therein.
It is the primary object of the present invention to provide an enhanced foam process for the production of non-woven webs that utilize SAP particles or fibers therein, and a system for practicing such a method. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.